4/08/2009 @ 6:00AM

Using Heat To Cool

Heat is valuable stuff. But as often as it’s used–to bake a cake or push a turbine around to generate electricity–it is vented into the ether, wasted.

As engineers grope for ways to make systems more efficient, their fat target is heat. Whether it’s generated intentionally, like in a power plant, or it’s a waste product, like from an internal combustion engine, researchers are finding ways to gather more of it and put it to work.

Researchers at Oregon State University are taking this quest to the micro scale. They are developing a small device that can attach to the exhaust systems of diesel generators used by the U.S. Army that will capture waste heat and use it to run air conditioners in the fields of battle (not so much for the soldiers’ comfort, but so the command’s electronics don’t fry).

“Waste heat is a more common resource than most people realize,” says Richard Peterson, a mechanical engineer and thermodynamics professor at Oregon State University. Peterson is developing the technology with Pacific Northwest National Laboratory through a partnership called the Microproducts Breakthrough Institute.

If successful, this kind of technology could be applied to every kind of motor. Long-haul trucks might find use for it first, but it could one day end up in your sedan.

The Army, which is funding the project, is hot for this kind of thing, because it pays more for fuel than even the Europeans do. It costs the Army between $200 and $400 a gallon to bring fuel to what it calls “forward-deployed” locations–the battlefield. There, the Army sets up mobile command centers powered by banks of diesel generators that produce electricity for communications equipment and so-called environmental control units, military-speak for air conditioners.

These air conditioners typically suck up about half of the electricity produced by these generators, depending, of course, on location and outside temperature. And the generators themselves, like all internal combustion engines, are hardly efficient. About 75% of the energy used to power the generator becomes waste heat. (Your car does much worse, however, since its motor has to rev up and down as you wind your way through town.)

Peterson is building a device that can bolt on to a generator’s exhaust system and capture that heat. It is a block of steel etched with what he calls “microchannel technology,” grooves about one-fourth of a millimeter wide through which a fluid called R245fa courses. The fluid gets hot, is then allowed to expand, and is used to run a compressor similar to one that runs a typical air conditioner.

This methodology, called an organic Rankine cycle, is being applied to all manner of systems, but most of them are big. Engineering giant
United Technologies
and geothermal company
Ormat Technologies
–its founder is credited with perfecting the cycle–are building generators that can employ low-grade heat to generate electricity from geothermal resources once thought to be not hot enough.

Commercial buildings and manufacturing facilities are installing cogeneration systems that use waste heat instead of venting it. And the most advanced gas power plants are combined-cycle plants that use two different thermodynamic cycles to capture different levels of heat.

But Peterson’s system is by far the smallest, and it could catch on. Peterson says using this system could reduce fuel consumption for the Army by 30%. Because the generators’ heat will be used to run the cooling systems, it frees up more electricity for the communications equipment, so fewer generators will be needed.

The system could be adapted for big rigs, too. Once the waste heat is captured by the fluid in the microchannels, it could be used for cooling, or to create electricity to help run the electronics in the truck’s cab.

Peterson has tested all of the components of the system separately. By this summer, he plans to have them all pieced together and bolted on to an Army generator to show what it can do.

If it works as hoped, the Army may adopt the technology. Peterson is working with colleagues to develop a cheap manufacturing process, but for now, the cold truth is that his heat-trapping system is far too expensive to commercialize.